1. Field
Aspects of the present invention relate to a Successive Ionic Layer Adsorption and Reaction (SILAR) process for depositing epitaxial ZnO on III-nitride based light emitting diode (LED), and an LED including the epitaxial ZnO layer.
2. Discussion of the Background
III-Nitride based LEDs are often fabricated using Mesa geometry. In the Mesa geometry, both the positive and negative electrical contacts to the LED are made on the top side of a semiconductor wafer including a p-type III-Nitride layer, an active layer, and an n-type III-Nitride layer disposed on a substrate. The sheet resistance of the p-type III-Nitride layer is generally much larger than the n-type III-Nitride layer. This causes a phenomenon known as p-contact current crowding. P-contact current crowding occurs because the path of least resistance for current flow results in higher electrical injection into the active layer of the LED in the vicinity of the external electrical contact to the p-type III-Nitride layer.
The resulting higher local current density and light generation near the p-contact leads to lower overall device efficiency. A typical solution to this issue has been the use of a current spreading layer to provide more uniform distribution of current injected by lowering the sheet resistance on the p-side of the device. Because the light generated must pass through the current spreading layer to escape the device, the current spreading layer typically includes layer(s) of a very thin semi-transparent metal or a transparent conductive oxide (TCO). Indium Tin Oxide (ITO) has become the industry standard material for current spreading layers in III-Nitride LEDs, due to having a good combination of transparency and electrical conductivity. However, ITO has high raw material costs, which make it generally undesirable. In addition, the highest quality ITO films are typically deposited using magnetron sputtering, which requires special precautions in order to prevent plasma damage to III-Nitride LEDs during deposition.
Zinc oxide based TCO films are used as an alternative to ITO in some applications. Zinc oxide may be deposited on III-Nitride LEDs by the same methods typically used for ITO films, including sputtering, but can also be deposited using low temperature aqueous solution deposition. Unlike ITO, ZnO also has a good crystal lattice match with GaN and other Wurtzite structured III-Nitride semiconductors with similar lattice parameters. The good lattice match allows for epitaxial ZnO layers to be formed on III-Nitride LED surfaces, even when the ZnO is formed by low temperature aqueous solution deposition. As compared to polycrystalline layers, epitaxial layers can possess higher optical transparency and electrical carrier mobility, which lead to enhanced current spreading layer performance. As compared to sputter deposition of ITO, the low temperature aqueous solution deposition of ZnO offers advantages in materials, capital equipment, and energy costs. This makes low temperature solution deposition of ZnO based TCO layers attractive for high-performance, low-cost, III-Nitride LED current spreading layers.
Aqueous solution methods have been used previously to synthesize a wide variety of ZnO films and Micro/Nano-Structures. In most cases, the deposition of a uniform film or array of nano/microstructures requires the use of a nucleation or seed layer. The purpose of the nucleation/seed layer is to provide a uniform distribution of sites for the growth of ZnO during the low temperature solution growth. FIGS. 1A and 1B show scanning electron microscope images of ZnO deposited on a c-plane GaN surfaces by low temperature aqueous solution deposition without the use of a nucleation/seed layer. As shown in FIGS. 1A and 1B, without a seed layer, conditions used for solution deposition/growth typically lead to non-uniform and/or low density of nucleation sites, which develop into a low density of spatially separated large structures or islands, rather than the desired uniform array or film.
Several different methods have previously been explored for nucleation/seed layer creation, including coating with a suspension of ZnO nanoparticles, coating with a precursor film which upon heating decomposes and crystallizes into ZnO, vapor deposition of a thin ZnO layer, and aqueous deposition by initiating the rapid precipitation of ZnO from solution. These techniques all have serious drawbacks for producing epitaxial films using low temperature aqueous solution deposition.
The use of nanoparticle seeds deposited from suspension is not compatible with epitaxial growth, as it creates a seed layer composed of particles with random orientations. The same is true for the precursor film method, unless very high temperatures are used to epitaxially recrystallize the initially polycrystalline ZnO seed layer. Vapor deposition is capable of producing epitaxial seed layers, but the use of such methods to produce the seed layer negates much of the cost advantage of using low temperature aqueous solution deposition for subsequent bulk film growth. Processes for creating the seed layer by precipitation from aqueous solution have been shown to create epitaxial ZnO, but the processes fail to provide high nucleation density and uniformity.
FIG. 2 shows a scanning electron microscope image of a seed layer produced by the precipitation method. The ZnO produced is visibly not uniform and leaves a significant portion of the GaN surface uncovered. In addition, the ZnO particles that simultaneously form during precipitation can settle on the seed layer surface in a random orientation and disrupt subsequent epitaxial growth. The majority of the zinc dissolved in the solution is consumed in the formation of powder particles rather than the seed layer, making the precipitation method of seed layer deposition very inefficient in precursor chemical use.
A Successive Ionic Layer Adsorption and Reaction (SILAR) method may be considered to be the aqueous solution phase analog to Atomic Layer Deposition (ALD). The SILAR process is performed by repeatedly cycling two self-limiting reactions, with each one adding the cationic or anionic atoms, respectively, to slowly build a binary compound film. ALD uses precursor molecules adsorbed from the gas phase. SILAR uses adsorption of ions dissolved in aqueous solution.
In a SILAR of oxides, water itself can be the source of the oxygen though hydrolysis and condensation reactions with the cationic species. Hydrolysis can be promoted though control of the pH, or simply using hot water. ZnO films can be deposited using a solution of zinc ammine complex ions to supply the Zn and hot water to perform the hydrolysis.
However, the conventional art has failed to provide a method of using SILAR to form a ZnO film formed on an LED. As such, the present invention is directed to providing a method of producing a ZnO film on an LED, using SILAR. Further, the present invention is also directed to providing LEDs that include such a ZnO film, which have improved characteristics over LEDs including conventionally produced ZnO layers.